Fulkoir only appears when a player on the quest Destroying the Foundation arrives in the gnoll camp area where he spawns. There doesn't appear to be any significant delay between his death and his respawn if new players arrive to kill him; he has been observed popping less than a minute after his death.

Though Fulkoir himself is not especially difficult to kill, beware the fast respawn of the surrounding monsters: Rockpaw scouts, Rockpaw guards and terra bats. If there aren't any when you show up, then somebody else has recently killed them.